Tube fittings are used to join or connect a tube end to another member, whether that other member be another tube end such as through T-fittings and elbow fittings, for example, or a device that needs to be in fluid communication with the tube end, such as for example, a valve. As used herein the terms “tube” and “tubing” are intended to include but not be limited to pipe as well. Any tube fitting must accomplish two important functions within the pressure, temperature and vibration criteria that the tube fitting is designed to meet. First, the tube fitting must grip the tube end so as to prevent loss of seal or tube blow out. Secondly, the tube fitting must maintain a primary seal against leakage. The requirement that a tube fitting accomplish these two functions has been the driving factor in tube fitting design for decades. A multitude of factors influence the design of a tube fitting to meet a desired grip and seal performance criteria, but basic to any tube fitting design will be: 1) the characteristics of the tubing that the fitting must work with, including the material, outside diameter and wall thickness; and 2) the tube grip and seal performance level required of the tube fitting for its intended applications. The goal is to design a tube fitting that reliably achieves the desired tube grip and seal functions within whatever cost constraints are imposed on the product by competing designs in the marketplace.
A flareless tube fitting generally refers to a type of tube fitting in which the tube end remains substantially tubular, in contrast to a flared tube fitting in which the tube end is outwardly flared over a fitting component. Flared tube ends are commonly encountered in use with plastic tubing and plastic tube fittings. The present invention is not directed to plastic tubing or tube fittings because such fittings have significantly different challenges and material properties that affect the ability of the fitting to both grip the tube and provide an adequate seal. Operating pressures and temperatures are also typically substantially lower in the plastics art. In other words, with respect to tube grip and seal, whatever works in a plastic tube fitting provides little or no guidance for a non-plastic tube fitting.
Tube fittings that are intended for use with stainless steel tubing, for example, are particularly challenging to design in order to achieve the desired tube grip and seal functions. This arises from the nature of stainless steel which, in terms of typical commercially available tubing material, is a very hard material, usually on the order of up to 200 Vickers. Stainless steel tubing is also used for high pressure applications in which the tubing wall thickness is substantial (referred to in the art as “heavy walled” tubing). Heavy wall tubing is difficult to grip because it is not only hard but it is also not particularly ductile. Low ductility makes it more difficult to deform the tubing plastically so as to achieve a desired tube grip.
Tube fittings for stainless steel tubing typically include an assembly of: 1) a tube gripping device, often in the form of a ferrule or ferrules, or a gripping ring-like structure, and 2) a pull-up mechanism for causing the tube gripping device to be installed on a tube end so as to grip the tube end and provide a seal against leakage. The term “pull-up” simply refers to the operation of tightening the tube fitting assembly so as to complete the assembly of the fitting onto the tube end with the desired tube grip and seal.
Usually a stainless steel tube fitting is first assembled in a “finger tight” condition and then a wrench or other suitable tool is used to tighten or “pull up” the fitting to its final initial and complete assembled condition. In some cases, especially for larger tube sizes, a swaging tool is used to pre-install a ferrule onto the tubing. The pull up mechanism most commonly used is a threaded connection of a female threaded nut component and a male threaded body component, with the tube gripping device being acted upon by these two components as they are threaded and tightened together. The body includes a tube end receiving bore with an angled camming surface at the outer portion of that bore. The most commonly used camming surfaces are frusto-conical such that the term “camming angle” refers to the cone angle of the camming surface relative to the tube end longitudinal axis or outer surface. The tube end is axially inserted into the body bore and extends past the frusto-conical camming surface. The gripping device is slipped onto the tube end and the nut is partially threaded onto the body to the finger tight position such that the tube gripping device captured axially between the camming surface and the nut. The nut typically includes an inward shoulder that drives the tube gripping device into engagement with the angled camming surface on the body as the nut and body components are threadably tightened together. The angled camming surface imparts a radial compression to the tube gripping device, forcing the tube gripping device into a gripping engagement with the tube end. The tube gripping device typically is to form a seal against the outer surface of the tubing and also against the angled camming surface.
The most commonly used tube gripping devices in stainless steel tube fittings today (the most commonly used are ferrule-type tube fittings) achieve tube grip by causing a front or nose portion of the tube gripping device to bite into the tube end outer surface. As used herein, the term “bite” refers to the plastic deformation of the tube gripping device into the outer surface of the tube end so as to plastically deform and indent the tubing with an almost cutting-like action to create a generally radial shoulder or wall at the front end of the tube gripping device. This “bite” thus serves as a strong structural feature to prevent tube blow out at high pressure, particularly for larger diameter tubing such as ½″ and higher.
Over the years there have been numerous tube fitting designs that do not rely on a “bite” type action, but rather merely radially compress the tube gripping device against the tubing outer surface, some with the effect of indenting into the tubing without creating a bite. These designs are not suitable for high pressure stainless steel tube fittings. The most common commercially available stainless steel tube fittings especially for high pressure applications have historically been of two radically distinct designs of the tube gripping device—single ferrule tube fittings and two ferrule tube fittings.
A single ferrule tube fitting, as the name implies, uses a single ferrule to accomplish both the tube grip and seal functions. However, it is becoming increasingly recognized that these two functions are at odds with each other when designing a tube fitting that can meet a desired tube grip and seal performance criteria. This is because the design criteria needed to assure that the tube fitting achieves an adequate tube grip usually works against the ability of the single ferrule to also provide an effective seal. Consequently, although prior art single ferrule fittings can achieve adequate tube grip in some cases, this tube grip performance comes at the expense of having a less effective seal. One result of this situation is that some single ferrule tube fittings have been designed with additional components and techniques to achieve an adequate seal. Less than optimum seal performance is particularly noted in single ferrule fittings that attempt to seal against gas, and especially high pressure gas. Single ferrule tube fittings thus are usually more suited to lower pressure liquid applications such as hydraulics, however, even in such lower pressure applications single ferrule seal performance remains less than desired.
For single ferrule tube fittings, the biting action is usually associated with the single ferrule being designed to bow in a radially outward direction from the tube wall in the central region or mid-portion of the single ferrule body between the front and back ends thereof. The front end of the ferrule is driven against the angled camming surface of the body by the nut pushing against the back end of the ferrule. The bowing action helps direct the front end of the single ferrule into the tube end. The bowing action is also used to cause the back end of the ferrule to likewise engage and grip the tube end. This is accomplished usually by provided an angled drive surface on the nut shoulder that engages the back end of the single ferrule so as to radially compress the back end of the ferrule into a gripping action on the tube end. In some single ferrule designs, the back end of the ferrule apparently is intended to bite into the tube end. This back end tube grip is sometimes used with the single ferrule in order to attempt to improve the tube fitting's performance under vibration because the back end grip attempts to isolate down-tube vibration from affecting the front end tube bite.
The use of a back end tube grip actually works against the effort to grip the tube end at the front end of the single ferrule. Ideally, the single ferrule should be completely in three dimensional compression between the nut and the camming surface of the body. Providing a back end grip actually places a counter acting tension to the single ferrule that works against the front end compression being used to provide the tube grip. Additionally, the outward bowing action tends to work against the effort to grip the tube at the front end of the single ferrule because, in order to enable the outward bowing action, the single ferrule requires a lessened mass that is adjacent the tube gripping “bite”. The outward bowing action radially displaces ferrule mass central to the ferrule body away from the tube end. Consequently, an outwardly bowed single ferrule fitting could be more susceptible to ferrule collapse, loss of seal and possibly tube blow out at higher pressures.
In order to achieve an adequate tube grip on stainless steel tubing, single ferrule stainless steel tube fittings have historically used a rather shallow camming angle of between ten and twenty degrees. This range of angles is referred to herein as “shallow” only as a term of convenience in that the angle is rather small. The shallow camming angle has been used in single ferrule fittings to obtain a mechanical advantage because the shallow angle provides an axially elongated camming surface against which to slide and radially compress the single ferrule front end to bite into the tube end outer surface. Hard stainless steel tubing material necessitated this elongated sliding camming action in order to be able to get the single ferrule to create an adequate bite for tube grip. Over the years, the single ferrule has been through hardened or case hardened so as to be significantly harder than the stainless steel tubing, however, the shallow camming angle is still used today in such single ferrule fittings to obtain a mechanical advantage from the ferrule sliding along the camming surface to produce the “bite” so as to assure an adequate tube grip. An example of a commercially available single ferrule tube fitting that uses a case hardened ferrule and a shallow camming angle of about twenty degrees is the CPI fitting line available from Parker-Hannifin Corporation. Another example is the EO fitting line available from Ermeto GmbH that uses a through hardened single ferrule and a twelve degree camming angle.
In some single ferrule designs, a non-conical camming surface has been tried whereby an attempt is made to simply press the ferrule against the outer surface of the tube end, thereby not creating a bite. The result in such cases however is a low grip or low pressure only fitting that are not well suited to stainless steel fittings.
The shallow camming angle and elongated camming surface and axial movement needed to achieve an adequate tube grip with a single ferrule fitting, however, compromises the ability of the single ferrule to achieve the seal function, especially in extreme environments and for sealing gas. This is because the front end of the single ferrule attempts to make the seal against the axially elongated camming surface. The radially outward bowing action causes a larger portion of the outer surface of the front end of the single ferrule to come into contact with the camming surface against which it is being driven. The result necessarily is a larger seal surface area between the outer surface of the single ferrule and the camming surface. This enlarged seal area causes an unwanted distribution of the sealing force between the single ferrule and the camming surface, and also creates a larger area for surface imperfections to allow leaks to occur. This is particularly a metal to metal seal issue (as contrasted to non-metal to non-metal seals: for example, in a plastic fitting it is usually desirable to provide an enlarged seal contact area because the more highly ductile plastic material can better form a seal between the two surfaces.)
Because historically the single ferrule fitting has used a shallow camming angle to achieve adequate tube grip, the less than optimum seal function is either tolerated as a recognized limitation on the application of the fitting, or additional features have been designed into the single ferrule fitting, most notably attempts to include one or more elastomeric seals with the single ferrule or with which the single ferrule cooperates to provide a better seal with stainless steel tubing. See, for example, U.S. Pat. Nos. 6,073,976 and 5,351,998. U.S. Pat. No. 6,073,976 illustrates a typical example of a single “ferrule” (called a “cutting ring” in the patent) fitting that attempts to solve the “seal” issue with added elastomeric seal. The U.S. Pat. No. 5,351,998 patent describes the benefits obtained by separating the tube grip and seal functions into two separate components.
A commercially available and highly successful two ferrule fitting used for tubing is commercially available from Swagelok Company, Solon, Ohio and is described in U.S. Pat. Nos. 6,131,963 and 3,103,373 both of which are owned in common by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference. In this two ferrule fitting, the tube grip and seal functions also are separately achieved by the use of two ferrules. The forward or front ferrule provides an excellent seal even against gas, and the back or rear ferrule provides an excellent tube grip.
The front ferrule achieves an excellent seal by camming against a shallow camming surface angle such as twenty degrees. This is because the front ferrule does not need to slide excessively on the camming surface in order to achieve a tube grip function. Likewise, the front ferrule is not case hardened because the primary purpose of the front ferrule is to seal and is not to bite into the tube end. Thus the relatively “softer” front ferrule achieves an excellent seal, particularly against gas, even though the body conical camming surface presents a camming angle of about twenty degrees.
The back ferrule achieves the tube grip function in the above noted two ferrule tube fitting. The back ferrule is case hardened to be substantially harder than the tube end. The front end of the back ferrule cams against a frusto-conical camming surface formed in the back end of the front ferrule. The ostensible angle of this camming surface is forty-five degrees, but due to the sliding movement of the front ferrule, the effective camming angle is actually a shallow angle of about fifteen to twenty degrees. Although the effective camming angle for the back ferrule is shallow, the back ferrule is not required to provide a primary seal (although it can form secondary or backup seals). The back ferrule also does not exhibit the undesired bowing action but rather grips the tube end as a function of a radially inward hinging action. As used herein, the term “hinging” refers to a controlled deformation of the ferrule such that a central region or mid-portion of the ferrule body undergoes an inwardly radial compression, as distinctly contrasted to a bowing or radially outward displacement. Thus, the effective shallow camming angle not only does not compromise the fitting seal capability, it actually substantially enhances the overall performance of the tube fitting especially for stainless steel tubing.
By using separate ferrules for each to achieve primarily only one of the key tube fitting functions, the two ferrule tube fitting achieves tremendous tube grip and seal functions. This prior art two ferrule tube fitting thus has enjoyed tremendous commercial success especially in the art of stainless steel tubing in part due to its performance characteristics such as high pressure rating on the order of 15000 psi, wide temperature rating of cryogenic to 1200° F. and in many applications a significant number of remakes (a “remake” refers to the loosening and re-tightening of a fitting after an initial pull-up).
U.S. Pat. No. 3,248,136 illustrates use of a single locking ring as opposed to a ferrule, wherein the locking ring acts against a surface having an angle that appears to be greater than twenty degrees or more, but the ring does not appear to plastically deform into the tubing but rather remains elastic so that the ring is designed to retain its original shape after pull-up, both of which are features that are unsuitable for stainless steel tube fittings of the type considered herein. Japanese utility model publication 44-29659 illustrates a tightening ring that appears to be intended to have a bowing effect and grip the tube at the front and back ends. The fitting does not appear to be applicable to stainless steel tubing as the tube is covered with a resin cover.
Many applications and uses of the described two ferrule tube fitting do not require such high pressure, temperature and remake performance characteristics. The present invention is directed to a new fitting concept that can meet lower performance characteristics without compromising overall fitting integrity and performance. Moreover, the present invention is directed to a new single ferrule tube fitting and variants thereof that can exhibit significantly improved performance over prior art single ferrule tube fittings, particularly for applications that do not require the high end performance characteristics of the above described two ferrule tube fitting.